Process for electrolytically coloring the anodically oxidized coating on aluminum or aluminum base alloys

ABSTRACT

In the process for electrolytically coloring an anodically oxidized coating on an aluminum or aluminum base alloy article by subjecting the anodized aluminum or aluminum base alloy article as a cathode to an electrolytic coloring treatment with a direct current in an electrolytic coloring bath containing a nickel salt, an improvement which comprises conducting the electrolytic coloring treatment at a pH in the range of from 2.0 to 5.5 to obtain a uniformly colored film.

Unite States atent Yanagida et al.

[ Dec. 30, 1975 PROCESS FOR ELECTROLYTICALLY COLORING THE ANODICALLYOXIDIZED COATING ON ALUMINUM OR ALUMINUM BASE ALLOYS Inventors: KiyomiYanagida; Tadashi Hirokane; Tadashi Tsukiyasu; Tomoari Sato, all ofNagoya, Japan 3,634,208 l/l972 Kuroda 204/35 N 3,759,801 9/1973 Patrie204/35 N 3,761,362 9/1973 Oida 204/35 N 3,788,956 1/1974 Pattie et al.204/35 N 3,798,137 3/1974 Kampert 204/35 N Primary Examiner-John H. MackAssistant Examiner-Aaron Weisstuch Attorney, Agent, or FirmSughrue,Rothwell, Mion, Zinn & Macpeak [57] ABSTRACT In the process forelectrolytically coloring an anodically oxidized coating on an aluminumor aluminum base alloy article by subjecting the anodized aluminum oraluminum base alloy article as a cathode to an electrolytic coloringtreatment with a direct current in an electrolytic-coloring bathcontaining a nickel salt, an improvement which comprises conducting theelectrolytic coloring treatment at a pH in the range of from 2.0 to 5.5to obtain a uniformly colored film.

8 Claims, No Drawings PROCESS FOR ELECTROLYTICALLY COLORING THEANODICALLY OXIDIZED COATING ON ALUMINUM OR ALUMINUM BASE ALLOYSBACKGROUND OF THE INVENTION 1. Field of the Invention The presentinvention relates to a process for coloring an anodically oxidizedcoating on an aluminum or aluminum base alloy article (hereinafterreferred to as aluminum for brevity). More particularly, the inventionrelates to a process for electrolytically coloring an aluminum articlewhich has previously been anodically oxidized, which comprisessubjecting the anodically oxidized aluminum as a cathode to anelectrolytic coloring treatment with a direct current in an electrolyticcoloring bath containing a nickel salt to obtain a uniformly coloredfilm.

2. Description of the Prior Art It was known that an electrolyticcoloring bath containing a nickel salt can provide uniformly coloredfilms in a stable manner at a low cost and, therefore, the electrolyticcoloring bath is quite suitable for the operation on an industrialscale. Furthermore, when an electrolytic coloring bath containing anickel salt is used, a wide variety of colors, i.e., from yellow brown,light brown to dark brown can be obtained and the colors obtained areexcellent in uniformity and color reproducibility.

However, when an aluminum article is electrolytically colored in theelectrolytic coloring bath containing a nickel salt on an industrialscale, a phenomenon sometimes occurs such that only a faint color isformed or the colored film tends to be peeled, i.e., so-calledoxide-spalling may occur, and thus it is difficult to obtain a uniformlycolored film on the aluminum articles in a stable manner.

SUMMARY OF THE INVENTION As the result of various investigations, it hasbeen found that the deviation in the pH value of the electrolyticcoloring bath is one of the main causes of the above-describeddifficulties. That is, if the pH value of the electrolytic coloring bathbecomes lower than 2.0, the color of the film on aluminum articlesbecomes quite faint and a sufficiently deep color which is practicallyuseful cannot be obtained even if the electrolytic coloring treatment iseffected for a prolonged period of time and, on the other hand, if thepH value of the electrolytic coloring bath becomes higher than 5.5, afilm having only faint color is obtained and, in addition, the filmformed tends to peel during the coloring treatment.

A primary object of this invention is to provide an improved process forelectrolytically coloring an anodically oxidized coating on an aluminumby subjecting the aluminum having the anodized oxide film in a thicknessof at least 6 microns thickness to a direct current electrolysis as acathode in an electrolytic coloring bath containing a nickel salt, theimprovement comprising conducting the electrolytic coloring treatment inan electrolytic coloring bath at a pH in the range of 2.0 to 5.5.

DETAILED DESCRIPTION OF THE INVENTION The purpose of the first anodicoxidation treatment of this invention is the formation of a practicallyuseful anodically oxidized coating on the surface of the aluminum and,in particular, the anodically oxidized coating I having a thickness ofat least 6 microns which has been formed on the aluminum in an anodicoxidation bath containing sulfuric acid and/or an aromatic sulfonic acidas a main component can be uniformly colored with the electrolyticcoloring treatment in a stable manner and further the colored film hashigh weatherability. For the formation of the anodically oxidizedcoating on the aluminum, an aqueous sulfuric acid solution having aconcentration of from about 10 to 30% by weight, perferably 10 to 20% byweight is usually used as the anodic oxidation bath and an aluminumarticle is subjected to the anodizing treatment with a direct currect atroom temperature (about 2030C) and a current density of about 1ampere/dm or, occasionally, at a higher current density of 3.0 to 5.0amperes/dm However, stable coloring treatment can be attained when theabove anodizing conditions, i.e., sulfuric acid concentration, currentdensity and bath temperature are changed to some extent, so long as thethickness of the anodically oxidized coating formed on the aluminumarticle is above 6 microns. Also, when an anodic oxidation bathcontaining an aromatic sulfonic acid as the main component is used, theanodization is preferably conducted in an aqueous solution of thearomatic sulfonic acid having a concentration of about 10% by weight bysuperimposing an alternating current on a direct current. i

The coloring treatment of the anodized article is then carried outwithout applying a sealing treatment by a direct current electrolysisusing the aluminum article as a cathode in an aqueous solutioncontaining a nickel salt as an electrolytic coloring bath. ,f

The electrolytic coloring bath used in this invention contains awater-soluble nickel salt as the main component and the electricconductivity of the bath can usually be controlled by adding thereto anappropriate amount of boric acid, sulfuric acid, etc. Examples of thewater-soluble nickel salt are nickel sulfate, nickel chloride and nickelacetate, and the concentration of the nickel ion as the main componentof the electrolytic coloring bath can vary over a wide range. Forexample, when nickel sulfate is used as the nickel salt, a desiredcolored film is obtained at a nickel sulfate concentration of 15 to I00g/liter. Of course, a satisfactory coloring can be attained using anelectrolytic coloring bath having nickel sulfate concentrations outsidethe above range, but, in this case, it becomes difficult andeconomically unprofitable to practice the electrolytic coloringoperation using an apparatus which is generally employed on anindustrial scale. Also, as described above, the electrolytic coloringbath used in this invention can contain further boric acid, preferablyat a concentration of 10 to 50 g/liter, for adjusting the electricconductivity of the bath whereby a more uniformly colored film can beobtained in a stable manner.

In the process of this invention, it is necessary that the pH of theelectrolytic coloring bath is maintained at 2.0 to 5.5 since when thecoloring treatment is 'conducted at a pH value outside the above range,the resulting color of the colored film becomes faint and the coloredfilm tends to be peeled and spalled. That is, the coloring treatmentcannot be conducted in a stable manner at a pH value outside the aboverange. Also, in a practical industrial operation, since a wide varietyof bronze-like colors can be obtained uniformly and with betterreproducibility by maintaining the pH of the electrolytic coloring bathin a range of 3.0 to 4.5, it is the electrolytic coloring bath in therange of from 3.0

The procedure for the electrolytic coloring treatment is advantageouslycarried out using a current density in a range of 0.05 to 3.0 amperesldmpreferably 0.1 to 2.0 amperesldm The bath temperature can be roomtemperature with a satisfactory result, but the coloring operation canbe conductedat a temperature in the range of from about 10 to about 40C.

Thetime required for the electrolytic coloring treatment can beappropriately selected depending upon the color tone desired. Generally,as the period of time of electrolysis increases, the color of the filmobtained becomes deeper. However, when a high current density such asabout 2.0 amperes/dm is employed in the electrolysis, a sufficientlydeep color is obtained in a short period of time, e.g., 2 to 5 secondsand when a low current density such as 0.1 to 0.3 ampere/dm is employed,a sufficiently deep color can be obtained at a comparatively longerperiod of time, e.g., l to 3 minutes.

On the other hand, from the practical standpoint, there is an inevitabletendency that the pH of the electrolytic coloring bath deviate from thepreferred range of 3.0 0 45, even from the range of 2.0 to 5.5, uponcontinuing the electrolytic coloring operation on an industrial scale.This is due to the contamination of the electrolytic coloring bath withan anodic oxidation bath th electrolytic coloring bath. Therefore, inorder to elim nate the above causes, it is required to strengthen thewater rinsing after the anodic oxidation or to sufficiently rinse thealuminum article from the anodic oxidation treatment and the supportmembers for the aluminum article by employing a water spray means and touse nickel having good solubility as the anode of the electrolyticcoloring cell. Moreover, if the pH of the electrolytic coloring bath isinadequately controlled, it is necessary to immediately adjust the pH ofthe electrolytic coloring bath to the pH within the above range. Thatis, when the pH of the electrolytic coloring bath becomes too high, thepH can be easily reduced by adding a small amount of sulfuric acid tothe electrolytic coloring bath and, on the other hand, when the pH ofthe electrolyticcoloring bath becomes too low, the ph can be increasedby dissolving a small amount of nickelhydroxide in the electrolyticcoloring bath or passing the electrolytic coloring bath through an anionexchange resin. Thus, the pH of the electrolytic coloring bath can bemaintained in the range of 2.0 to 5.5, preferably 3.0 to 4.5.

Procedures for increasing the pH of the electrolytic coloring bath willbe described below in greater detail.

In the first procedure, the pH of the electrolytic coloring bath can beincreased by dissolving nickel hydroxide as a neutralizing agent in thebath. Nickel hydroxide used in this procedure can be in the form of apowder or a paste. Generally, a small amount of nickel hydroxide will besufficient. The neutralizing agent used in this procedure is not limitedto nickel hydroxide, but on considering the influences of various metalions on the color of the colored film, the use of a nickel compound is.preferred. Also, this procedure usually exhibits satisfactory resultsusing nickel hydroxide at a 'concentration,ofQfrom-lO to 1000 ppm. and,therefore, the pH control can be effected conveniently at low cost.

However, in practicing the neutralization, addition of alkalis which-aregenerally used in the conventional neutralization procedure such assodium hydroxide, potassium hydroxide, aqueous ammonia, etc. should beavoided in this invention since the contamination of the electrolyticcoloring bath with alkali metal ions such as sodium ion, potassium ionor the like sometimes results in the spalling of the colored film onaluminum during the coloring treatment thereby adversely affecting thequality of the colored film. Further, when ammonium ions are present inthe electrolytic coloring bath, a deep color cannot easily be obtainedwhereby the color is undesirably limited to specific colors.

In the second procedure, the pH of the electrolytic coloring bath whichhas decreased below 2.0 can be increased by treating the coloring bathwith an anion exchange resin. In this case, the use of a weak basicanion exchange resin is most preferable since free sulfuric acid ismainly removed by the anionexchange resin and thus the pH of theelectrolytic coloring bath can be readily adjusted to a value in thedesired pH range.

The invention will be further explained in greater detail by referringto the following Examples but these Examples are not to be construed aslimiting the scope of this invention. Unless otherwise indicated, allparts, percents, ratios and the like are by weight.

EXAMPLE I An aluminum plate (99.2% Al) was immersed in an aqueous 10%sodium hydroxide solution at 60C for 2 minutes, and then subjected to aneutralization treatment with a 20% aqueous nitric acid solution at roomtemperature. After rinsing the aluminum plate with water, it wassubjected to an anodic oxidation treatment in a aqueous sulfuric acidsolution for 15 minutes at a current density of 2 amperes/dm and at abath temperature of 1C. The anodized aluminum plate thus obtained wasthen subjected to electrolytic coloring treatment as a cathode with adirect current in an electrolytic coloring bath containing 50 g/liter ofnickel sulfate and g/liter ofboric acid for 1 minute at a currentdensity of 0.3 ampere/dm and at a bath temperature of 20 lC using anickel plate as an anode. In this case, the pH of the above electrolyticcoloring bath was reduced to 1.5 using sulfuric acid and then the pH ofthe electrolytic coloring bath was gradually increased to 5.7 bydissolving nickel hydroxide therein, whereby the electrolytic coloringtreatment was conducted at various pH values of the bath.

The color lightness of the resulting aluminum plates which have beencolored at various pH values of the electrolytic coloring bath was thenevaluated using an integrating automatic colorimetric color-differencemeter type AU-SCH-2 (made by Toyo Rika Kogyo K.l(.), and the resultsobtained are shown in Table l. The values in Table 1 below are shown interms of the lightness in the Munsell color system (Vy). When the colorof the bronze-like colored film on the aluminum is shown in terms of thevalue in the Munsell color system, a value above5.0 indicates the colorof aluminum itself and a value in the range of from 4.0 to 5.0

indicates a very faint color tone. At a value below 4.0, the color ofthe film becomes deeper as the value lowers, and, at about 1.5, thecolor is very deep.

6 What is claimed is: 1. In a process for electrolytically coloring ananodically oxidized coating on an aluminum or aluminum Table 1 pH 1.52.0 2.3 2.6 3.0 3.5 4.1 4.5 4.9 5.4 5.7 Lightness The colored film wasspalled at the pH of 5.7.

As is clear from the results shown in Table 1, it is necessary that thepH of the electrolytic coloring bath be in the range of 2.0 to 5.5.

EXAMPLE 2 alloy article comprising subjecting said article to anodicoxidation in an aqueous anodic oxidation bath consisting essentially ofsulfuric acid or an aromatic sulfonic acid to form an oxidized film of athickness of at least 6 microns and electrolyzing said anodic oxi- Analuminum plate (99.2% Al) was subjected to the dized article as acathode immersed in an aqueous same pre-treatment and anodic oxidationtreatment as electrolytic coloring bath containing a water-soluble inExample 1, and then subjected to an electrolytic nickel salt selectedfrom the group consisting of nickel coloring treatment with a directcurrent in an electrosulfate, nickel chloride and nickel acetate using adirect lytic coloring bath containing 35 g/liter of nickel sulfatecurrent, the improvement of obtaining uniformly coland 35 g/liter ofboric acid for 2 minutes at a current I ored coating onthe aluminumarticle by maintaining density of 0.3 ampere/dm and at a bathtemperature of the pH of said electrolytic coloring bath in a range of20 i 1C using the anodized aluminum plate as a cathfrom 2.0 to 5.5during said electrolytic coloring treatode and a nickel plate as ananode. In this case, the ment. electrolytic coloring was effected usingthe electrolytic 2. The process according to claim 1, wherein saidcoloring bath having a pH of 15 adjusted with sulfuric nickel salt isnickel sulfate. acid, and then the electrolytic coloring bath was passed3. The process according to claim 1, wherein said through a weak basicanion exchange resin, Duolite-A6 electrolytic coloring bath furthercontains boric acid. (Registered Trade Mark: Diamond Shamrock Corp.) at4. The process according to claim 1, wherein the pH various spacevelocities to increase the pH of the bath of the electrolytic coloringbath is maintained at 3.0 to whereby the electrolytic coloring treatmentwas per- 4.5 during said electrolytic coloring treatment. formed atvarious pH values of the electrolytic coloring 5. The process accordingto claim 1, wherein the pH bath. The color lightness of the resultingcolored alumiof the electrolytic coloring bath is maintained at 2.0 tonum plates was evaluated as described in Example 1, 5.5 by adding asmall amount of sulfuric acid to the and the results obtained are shownin Table 2 below. bath.

Table 2 pH 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 5.7 Lightness The coloredfilm was spalled at the pH of 5.7.

As is clear from the results shown in Table 2, the 6. The processaccoding to claim 1, wherein the pH suitable pH value ranges from 2.0 to5.5, and more, of the electrolytic coloring bath is maintained at 2.0 topreferably from 3.0 to 4.5. In addition, it was con- 5.5 by addingasmallamount of nickel hydroxide to the firmed that the concentrations of thenickel sulfate and bath. boric acid in the electrolytic coloring bathare not sub- 7. The process according to claim 1, wherein the pHstantially influenced by passing the coloring bath of the electrolyticcoloring bath is maintained at 2.0 to through the ion exchange resin.5.5 by passing the electrolytic coloring bath through an I While theinvention has been described in detail and anion exchange resin. withreference to specific embodiments thereof, it will 8. The processaccording to claim 7, wherein said be apparent to one skilled in the artthat various anion exchange resin is a weak basic anion exchange changesand modifications can be made therein withresin. out departing from thespirit and scope thereof.

1. IN A PROCESS FOR ELECTROLYTICALLY COLORING AN ANODICALLY OXIDIZEDCOATING ON AN ALUMINUM OR ALUMINUM ALLOY ARTICLE COMPRISING SUBJECTINGSAID ARTICLE TO ANODIC OXIDATION IN AN AQUEOUS ANODIC OXIDATION BATHCONSISTING ESSENTIALLY OF SULFURIC ACID OR AN AROMATIC SULFONIC ACID TOFORM AN OXIDIZED FILM OF A THICKNESS OF AT LEAST 6 MICRONS ANDELECTROLYZING SAID ANODIC OXIDIZED ARTIC;E AS A CATHODE IMMERSED IN ANAQUEOUS ELECTROLYTIC COLORING BATH CONTAINING A WATER-SOLUBLE NICKELSALE SELECTED FROM THE GROUP CONSISTING OF NICKEL SULFATE, NICKELCHLORIDE AND NICKEL ACETATE USING A DIRECT CURRENT, THE IMPROVEMENT OFOBTAINING UNIFORMLY COLORED COATING ON THE ALUMINUM ARTICLE BYMAINTAINING THE PH OF SAID ELECTROLYTIC COLORING BATH IN A RANGE OF FROM2.0 TO 5.5 DURING SAID ELECTROLYTIC COLORING TREATMENT.
 2. The processaccording to claim 1, wherein said nickel salt is nickel sulfate.
 3. Theprocess according to claim 1, wherein said electrolytic coloring bathfurther contains boric acid.
 4. The process according to claim 1,wherein the pH of the electrolytic coloring bath is maintained at 3.0 to4.5 during said electrolytic coloring treatment.
 5. The processaccording to claim 1, wherein the pH of the electrolytic coloring bathis maintained at 2.0 to 5.5 by adding a small amount of sulfuric acid tothe bath.
 6. The process accoding to claim 1, wherein the pH of theelectrolytic coloring bath is maintained at 2.0 to 5.5 by adding a smallamount of nickel hydrOxide to the bath.
 7. The process according toclaim 1, wherein the pH of the electrolytic coloring bath is maintainedat 2.0 to 5.5 by passing the electrolytic coloring bath through an anionexchange resin.
 8. The process according to claim 7, wherein said anionexchange resin is a weak basic anion exchange resin.